net: http: server: Select POSIX_C_LIB_EXT instead of FNMATCH

The CONFIG_POSIX_C_LIB_EXT will get support for fnmatch() function.
The old CONFIG_FNMATCH is deprecated.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>
(cherry picked from commit 0f90affc)